The present invention relates to mapping the electrical resistivity distribution in a subsurface, particularly to electrical resistance tomography for mapping subsurface electrical resistivity distribution, and more particularly to a method which involves electrical resistance tomography using pairs of steel cased boreholes as electrodes to obtain data which is used in an inverse calculation to obtain a model of the electrical resistivity distribution in the subsurface.
Boreholes are drilled for many purposes. Many of these boreholes are cased with pipes or casings constructed of various materials including plastics and steel. It is often desired to know the subsurface properties near these boreholes and many techniques have been developed to provide this information. A more difficult problem, but one that is often just as important is to be able to determine the subsurface properties at some distance from the borehole. Again, methods have been developed and new methods are currently being developed for this use, especially for the need to interrogate the plane between the boreholes. Electrical resistivity tomography (ERT) is one of the methods which has been developed to satisfy this need. ERT is a method for determining the electrical resistivity distribution in a volume from discrete measurements of current and voltage made within the volume or on its surface. Resistivity survey have been typically performed using electrodes embedded in the soil or in a borehole. These prior approaches are exemplified by U.S. Pat Nos. 4,796,186; 4,820,989; and 4,882,542; and by W. Daily et al., "Cross-Borehole Resistivity Tomography", Geophysics, Vol. 56, No. 8 (August 1991), pp. 1228-1235; W. Daily et al., "Electrical Resistivity Tomography of Vadose Water Movement", Water Resources Research, Vol. 28, No. 5, pp. 1429-1442, May 1992; and C. J. Schenkel, "Resistivity Imaging Using A Steel Cased Well", UCRL-JC-121653, June 1995.
The present invention involves electrical resistance tomography (ERT) using steel cased boreholes as electrodes, instead of the conventional electrodes previously used to perform ERT. Typically, the conventional electrodes are several inches in length and a few inches wide; in contrast steel downhole casings are ten to thousands of feet in length and several (4-8) inches in diameter. By the use of existing steel borehole casings, or conductive metal liner, the need for additional drilling to emplace deep electrodes, and the associated cost, is reduced or eliminated depending on the number and location of the existing steel cased or conductive metal lined boreholes. Pairs of casings or liners are used as current injection electrodes and as potential measurement electrodes to generate data which sample the subsurface resistivity distribution. This data is used in an inverse calculation to obtain a model of the electrical resistivity distribution. The subsurface model can be two or three dimensional in resistivity depending on the detail desired in the calculated resistivity distribution.